The present invention is directed to bicycle transmissions and, more particularly, to a bicycle hub transmission using a planet gear mechanism for switching gear ratios.
Small bicycles (referred to as "BMX bicycles") having 20-inch wheels are used in off-road motocross racing. A fast start is an important element of BMX motocross racing. Thus, downshifting to a slightly lighter gear ratio than during normal riding is recommended during startup in order to achieve higher startup acceleration. For this reason, BMX bicycles used in motocross racing are equipped with an external shifter comprising a rear derailleur and a hub cog having two cross-ratio sprockets, wherein a shifting lever is linked by a cable to the rear derailleur. Installing an external shifter, however, makes it necessary for the rider to control this shifter by operating a shifting lever. Since BMX motocross racing involves navigating a course having a succession of small bumps and curves, the rider is busy steering and does not have much time for shifting. Consequently, it is very difficult to shift gears during a race.
To overcome this problem, the rear wheel may be equipped with an internal shifter hub, and the gears may be shifted automatically by changing the speed of the internal shifter hub in accordance with centrifugal force created by the rotating wheel as shown in U.S. Pat. No. 3,603,178. Such an internal shifter hub comprises a hub axle, a driver capable of rotating about the hub axle, a hub shell, a planet gear mechanism for changing the speed of rotation of the driver and transmitting the result to the hub shell, a clutch mechanism for transmitting the output of the planet gear mechanism to the hub shell or stopping such transmission, a clutch-switching mechanism for switching the clutch mechanism by centrifugal force, and a support sleeve disposed between the planet gear mechanism and the hub shell.
The planet gear mechanism comprises an inner-tooth gear, a sun gear, a plurality of planet gears for meshing with the inner-tooth gear and the sun gear, and a planet gear carrier for supporting the plurality of planet gears. The clutch-switching mechanism comprises a weight member that swings by centrifugal force and a control member that rotates in response to swinging of the weight member. The weight member and control member are mounted on a weight support, itself mounted rotatably on the support sleeve. The weight support is linked to the inner-tooth gear, and the clutch mechanism has a rocking clutch pawl that is housed in the weight support.
In operation, the clutch mechanism is kept in a disengaged state by the clutch-switching mechanism, the rotation of the driver is transmitted to the hub shell via the carrier and the support sleeve, and the hub shell is directly driven until a prescribed centrifugal force is produced. The weight member of the clutch-switching mechanism is swung and the control member is switched from the disengaged state to a linked state when the hub is rotated and the centrifugal force surpasses a prescribed level. When this happens, the rotation of the driver is outputted to the weight support via the carrier, planet gears, and inner-tooth gear, power is transmitted from the weight support to the hub shell via the clutch mechanism, and the hub shell is driven in an upshifted mode.
In the conventional structure described above, the bicycle is driven in an upshifted mode through the intermediary of a planet gear mechanism during normal riding because of a switch from directly coupled driving to upshifted driving. In the planet gear mechanism, the power transmission efficiency is lowered because power is transmitted via a plurality of gears. Consequently, the conventional structure described above performs upshifted driving during normal riding, and the power transmission efficiency during normal riding is therefore lowered in comparison with the efficiency achieved at the beginning of the ride.
To increase the power transmission efficiency during normal riding, downshifting should be performed with the aid of the planet gear mechanism during startup, and the driver and the hub shell should be directly coupled once the centrifugal force has exceeded a prescribed level. Two power transmission paths should be formed in order to realize this concept: a downshifted path for transmitting rotation from the driver to the hub shell via the inner-tooth gear, planet gears, and carrier; and a direct-coupled path for transmitting rotation from the driver directly to the hub shell. These two paths should be switched by the clutch mechanism. In view of this, a structure has been proposed in which the weight support of the conventional internal shifter hub described above is nonrotatably linked to the hub shell, a clutch mechanism is provided between the driver and the weight support, and the weight support is linked to the carrier. In this structure, power is transmitted to the hub shell via the driver, inner-tooth gear, planet gears, carrier, and weight support in the case of the downshifted path, and via the driver, control member, and weight support in the case of the direct-coupled path.
The weight support and the hub shell are commonly joined by welding, riveting, or another fixing means. When the weight support and hub shell are linked by such a fixing means, they can be easily aligned, and the alignment accuracy does not deteriorate. The rotation balance of the two members rotating at a high speed is therefore difficult to disrupt. It is, however, impossible to take apart the two members linked by such a fixing means. The inability to disassemble the weight member and the hub shell makes it more difficult to replace or adjust the weight member. This, in turn, makes it impossible to alter the shift timing or to adjust it in accordance with the course profile or rider preferences during a motocross.
In view of this, a proposal has been made to link the hub shell and the weight member with the aid of a plurality of bolts. When the weight member and the hub shell are linked with the aid of a plurality of bolts, however, much labor is involved in tightening the bolts during assembly or in removing them during disassembly, thus making it necessary to align and link the two members each time they are assembled and further complicating the assembly operations.
A structure in which the hub shell and the slave cylinder of a "free hub" type of bicycle transmission are fastened with the aid of a cylindrical bolt is described in Japanese Utility-Model Publication 1-13605 as a possible solution for a bicycle free hub. Linking the two members with such a single cylindrical bolt facilitates disassembly and assembly and makes it possible to align the two members.